Experimental systems involving cocultivation of two cell types and somatic cell hybridization are employed to study the genetic control of melanogenesis and to study cell interactions on growth and differentiation of melanocytes in culture. These studies can provide information about the mutual behavior of cells in the multicellular organism. A transformation of normal into cancer cells probably also involves a changes in cell interaction. Cancer immunology involves another aspect of cell interaction, that is, between host cells and tumor cells. A careful analysis of the fundamentals of the genetics of melanogenesis and the effects of cell interaction on growth and differentiation of melanocytes is essential for the understanding of abnormal pigment cell growth and pigmentary disorders.